The present invention relates to a temperature compensated electronic timepiece having a temperature compensation circuit which is implemented as a monolithic integrated circuit formed of MOS transistors.
Hitherto, there have been a number of proposals for methods of performing temperature compensation of a standard frequency signal source in an electronic timepiece by sensing the ambient temperature, utilizing a temperature sensing circuit mounted in the timepiece. However, such proposals have not been put into practice to any considerable degree, due to such factors as the increased number of components required, and complexity of adjustment. However in recent years, timepiece users have become aware of the high degree of accuracy that can be obtained from an electronic timepiece employing a quartz crystal oscillator circuit. Thus, if a user owns a number of such timepieces, he will expect the same degree of timekeeping accuracy from a timepiece which is actually worn as from a timepiece which is not worn, but for example is left inside a desk. In addition, a high degree of accuracy will be expected even if a timepiece is left unused for a long period of time. In view of such requirements, temperature compensation will be necessary, and there is therefore a requirement for a low-cost temperature compensation circuit to be developed.
Prior art methods of temperature compensation include the capacitor temperature compensation method, and the method of using an expensive AT-cut quartz crystal vibrator which has a good frequency/temperature characteristic in a high-frequency oscillator circuit used as a timebase signal source. In addition, a method has recently been developed whereby two low-frequency tuning-fork type quartz crystal vibrator elements having different temperature characteristics are used to provide temperature compensation in an electronic timepiece. However, with the latter new method, the temperature compensation range is narrow, i.e. from 0.degree. C. to 40.degree. C., while if it is attempted to provide a wider range of temperature compensation, then as a result of the need to apply a close degree of compensation control to the frequency divider circuit of the timepiece, the temperature compensation cycle time (i.e. the duration of each repetitively performed temperature compensation operation) will become excessively long. Thus, this method has the disadvantage that it is necessary to use special measurement equipment in order to measure the timekeeping rate accuracy. Generally speaking, the gate time of the generally used type of timekeeping rate measurement equipment can be selected to be in the order of 2 seconds, or 4 seconds, or in some cases as much as 10 seconds, and the temperature compensation operation cycle time of a timepiece whose timekeeping rate is to be measured must match this gate time of the rate measurement equipment. If this cannot be achieved, then considerable inconvenience will result, with regard to after-sales service, and due to the fact that the timepiece will not be suitable for mass production manufacture. In addition, in the case of a temperature compensation method which uses a frequency difference between two quartz crystal vibrator elements, noise can result due to mutual interference between the two oscillator circuits. In order to avoid this, the circuits must be spaced a substantial distance apart, and as a result, the two oscillator circuits will not reach a common temperature equilibrium state. Thus, temperature data produced thereby may be erroneous.
Another problem which has arisen is with regard to the method of controlling the timekeeping rate of the timepiece in accordance with some measured compensation quantity which varies with temperature. One method is to control the frequency of oscillation of a quartz crystal oscillator circuit used as standard frequency signal source, by periodically connecting a capacitor to this oscillator circuit during time intervals whose duration (i.e. duty cycle) is controlled in accordance with the compensation quantity. However although such a method provides extremely precise control of the timekeeping rate, the maximum amount of temperature compensation that can be achieved is determined by the amount of capacitance which can be switched in this away. In order to make the range of temperature compensation sufficiently wide, it is necessary to make the amount of switched capacitance large. However if this is done, then the optimum conditions for operation of the quartz crystal oscillator circuit cannot be met, so that it is not possible to simultaneously provide optimum operation of the quartz crystal oscillator circuit and also provide a wide range of temperature compensation.
It is an objective of the present invention to overcome the problems described above, and to provide temperature compensation over a wider temperature range than has been possible in the prior art, and to provide a temperature compensated electronic timepiece whose timekeeping rate measurement and adjustment can be carried out using timekeeping rate measurement equipment in the same way as a conventional electronic timepiece, at the time of dispatch from the factory or in a retail store. Such a timepiece is easy to use, and highly suited to mass production manufacture.